I am playing as a soothsayer and the destruction skill says that mages who specializes in a combat spell inflict greater damage. not sure what it means?? as in it is only useful when i have mages? in my troops or any spell that i cast?

It affects spells you cast. For example if you have a Poison skull spell and no destruction skill, your spell will do X - Y damage. Let's say 1000 - 3000. With destruction skill on 3 you get 50 % bonus, so it becomes 1500 - 4500 damage.I'm unsure however if spells cast by your magicians are also affected. Especially the ones from the Book of evil, which casts in fact the spells you can cast as well.